Transistors and printed circuit boards came into use in various electrical systems nearly simultaneously. They provided a natural combination of components which added greatly to the miniaturization of electronic circuitry.
The connection of the printed circuit board with other parts of the electrical system with which it was used was first accomplished by simply soldering wires on the board and connecting them to a wiring plane or another printed circuit board. Connectors were soon developed which consisted of a connector blade soldered to the printed circuit board, with the blade making contact with another connector blade on a wiring plane or on another printed circuit board. These connectors were arranged as male or female connectors, as the need dictated.
As semiconductor technology advanced, so did the density of the various components contained on a given printed circuit board. Before long, it became apparent that considerable money could be saved if half of the mating connectors could be eliminated, in particular the part that was located on the printed circuit board. Significant strides were made at that time in arranging the printed circuit board to be a male member of a mating contact. These contacts were then arranged along the edge of the printed circuit board, and the board was inserted in the female side of the connector.
The advance of semiconductor technology tended to remove more and more discrete components and place them on integrated circuit chips. The use of such chips led to printed circuit boards of increasing complexity in function. However, as the number of external connections with various components of the printed circuit board increased, there was a limit on the maximum number of such connections that is possible with a connector of a given size.
As a consequence, efforts have been made to increase the "density" of printed terminals on the printed circuit board both by decreasing the width of each terminal and by decreasing the spacing between adjacent terminals. Such efforts are limited by the fact that if the terminals are made too narrow, there is a substantial loss of connector current capacity by reason of the reduced area of contact, and in addition it is difficult to assure proper alignment between the narrower terminals and their associated contacts within the card edge connector. There is also an irreduceable minimum spacing that must be maintained between terminals.
The most recent method of increasing density is to install a separate connector on the printed circuit board in lieu of printed terminals, which doubles the cost with only a slight reduction of insertion force. This expedient amounts to providing two mating connections where there was previously only one.
Various other attempts have been made to increase the number of printed terminals on a printed circuit board that can be accommodated in card edge connectors by changing the structure of the connector itself. However, all attempts of this kind of which applicant is aware have had some shortcoming.
One of these approaches has been to employ stepped terminals on the printed circuit board and correspondingly stepped contacts in the connector, as in Japanese laid-open document No. 58-70688. Such a connector is expensive to make, requires a thicker printed circuit board than is ordinarily used, and does nothing to meet the problem of high insertion and retraction forces that will be discussed below.
Another attempt to increase the density of the printed terminals with which the connector can be used utilizes two rows of printed terminals, with the terminals staggered as one moves alternately along one row and then the other. The contacts in the connector are similarly staggered to match up with the pattern of the terminals. Examples of this type of connector are shown in Japanese laid-open documents Nos. 51-162966, 55-8212 and 55-38411. As will be seen from FIG. 3 of the first mentioned document, this approach results in an increase of only a fraction of the total number of terminals that can be accommodated by the connector.
A third approach sometimes doubles the density of the printed terminals on the printed circuit board, but in every case produces another troublesome problem --unwanted and dangerous contacts between unmatched printed terminals on the circuit board and contacts within the card edge connector. Examples of such connectors are disclosed in Japanese laid-open documents Nos. 53-132654, 56-61777, 57-69795 and 58-188995.
In addition to the indicated spacing constraints, conventional electrical connectors of the male/female type have presented another problem. Such card edge connectors presently in use must maintain relatively high contact pressure between the terminals on the printed circuit board and the contacts connected with the output leads of the connector, because they supply the only force holding the connector blades against the printed circuit board. In fact, the pressure that is required to be maintained between the printed terminals of a printed circuit board and the associated contacts within the card edge connector is often so high that the circuit board can not be inserted within the connector simply by being pushed in, but must actually be hammered in by the user of the system.
The pressure between a printed circuit board and the contacts within a card edge connector that causes the board to be inserted into the connector only with extreme difficulty makes it nearly as difficult to extract the board from the connector. This difficulty is further compounded by the fact that generally neither the connector nor the printed circuit board can be grasped conveniently to exert the necessary extration force, and various tools have had to be developed to assist in applying such force to pry to the two members apart.
Zero insertion force and low insertion force connectors are known, but many of them are unnecessarily complicated in structure and none meets the problem of printed terminal density discussed above.
Applicant's invention meets both the problems discussed, by (1) making possible a greatly increased number of printed terminals on the printed circuit board that can be accommodated by this connector, and (2) achieving a secure mode of connection without having to employ the very high contact pressure that is required with conventional connectors.